This is a method of downsizing the rotary engine for highway cruising. Right now the RX8 engine is running at about 3250 RPM at 65 MPH and getting 25 MPG. My guess it is using about 30 HP to go 65 MPH. That would be a fuel burn of around 2.6 gallons an hour or 15.6 pounds per hour or a BSFC of around .52. BSFC is defined as the number of pounds of fuel burned for every HP generated in one hour. Here is a very old BSFC map from a NSU Wankel rotary engine. No doubt the RX8 engine BSFC is considerable improved over this engine never the less the basic principles still apply. I don’t have a corresponding map for the RX8 engine in case you were wondering.

An RX8 engine running at the best BSFC point should be around .47 according to the measurements some have made in aircraft service. This high BSFC problem in common with all cars. The best BSFC point for the engine is rarely at highway speeds or highway RPMs. The main reason smaller engines get better MPG on the highway is the small engines are running in a higher efficiency levels. Namely wider throttle openings.

Putting smaller engines in cars with higher axle ratios (lower engine RPM) to improve the MPG is called down sizing and down speeding. So a smaller rotary engine would get better MPG in an RX8 car at steady speeds of around 65 MPH.

A turbo compound rotary could achieve a BSFC of about .38. Turbo compound engines use small turbines extracting HP from the 50% waste energy in a gallon of gas and feeding it back into the output shaft. A well known technology from the 1950’s used in airliner piston engines. One of the problems with these A/C engines was the failure of an exhaust valve would take out the turbine. Needless to say the rotary has no exhaust valves. The 8 HP turbine would be geared down by at least ten to one. Working backwards a BSFC of .38 would be a fuel burn for 30 HP of 11.4 pounds per hour or 1.9 gallons per hour or a MPG of 34.2 MPG at 65 MPH.

The trouble with turbo compounding a 240 peak HP engine while running at 30 HP is it does not appear to work. What we need is a small rotary running at 80% of peak power or more to achieve a BSFC of .38 with a turbo compound configuration. The solutions is to add a small turbo compounded rotor to the front of the current engine.

One of the unique features of a wankel engine is the possibility of drilling a straight hole through the output shaft from end to end. If we design a small single rotor turbo compound engine of around 50 HP we can achieve 35 MPG on the highway at a steady speed of 65 MPH.

What you want to do is stop the not needed rotors as they consume friction HP. This beats the current Daimler/Chrysler 4-8 engine as the deactivated pistons don’t have to go
along for a free ride. Less overall engine friction.This engine can feed its power through the large two rotor engine shaft while the two rotor engine is shut down. The large engine is only needed for acceleration and climbing hills. All accessories such as water pump and alternator would be driven by the small engine.

Approaching this from another direction. My guess on the frontal area of the RX8 is 25 square feet. Five feet high and six feet wide by .8 (old rule of thumb). The C sub d is around .3 so flat plate drag is 7.5 square feet. Aero drag will be 7.5 X .0026 X 80 MPH ^2
or 125 pounds. Drag HP will then be: (117 FPS X 125 ) / 550 ft/lb sec. or 25.5 HP.
Give it another 11 HP for rolling drag, gear eff. and accessories. The attached charts are some measurements I made of total RX8 drag and HP required using the coast down technique so this number correlates rather well.

Since we now know the optimized BSFC of the leaned rotary engine is .47 and a car requires 36 HP at a steady 80 MPH and if we optimized the SIZE of the rotary engine for cruise at 80 MPH and 36 HP we should be burning 17 pounds per hour or 2.8 GPH. That equates to 31 MPG. Right now the RX8 is getting about 20 MPG at a steady 80 MPH. I have a Car Chip to measure it at 80 MPH but I haven’t got around to measure that accurately yet.

http://www.davisnet.com/drive/products/carchip_products.asp

I have measured it at 55 MPH, 65 MPH and 70 MPH. It appears to be about 25 MPG in this range. The MPG numbers assumed the mixture was set at 14.7:1 which I think the RX8 computer maintains. To calculate fuel burn you take the pounds of air burned per minute from these charts and calculate the fuel required.

So I am guessing that down sizing the rotary engine is worth 11 MPG at 80 MPH. The turbo compound feature will only have to provide an additional 4 MPG to achieve our goal of a 35 MPG RX8 at a steady 80 MPH. Four MPG is worth a lot in these days of $3 US a gallon gas.
In the rest of the world where gas is $5 or $6 a gallon it should be worth a lot more.

BTW this approach results in no loss of RX8 acceleration or top speed. In fact both may be increased. It is not a free lunch however as the cost of the engine will be increased.

Below are links to the official Mazda RX8 brochures for the 2004-2008 model year, which contain pictures and codes for all the parts in a RX8. These manuals are very handy, for instance, when trying to change/repair a part of your car :

The Dynamic Stability Control (DSC) system on the car relates your steering inputs to the actual motion/slippage of the wheels, and attempts to bring the car back under control by adjusting both engine torque and braking. The DSC cannot perform “magic” and break the laws of physics however.The DSC needs to be recalibrated after the battery is disconnected so that it can operate correctly. If not calibrated then it will noticeably interfere with your driving experience. Any dealer that you take the car to should be aware of this and perform this for you if they disconnect the battery.

If you disconnect the battery yourself, there should be a flashing “DSC OFF” message on the instrument cluster and a permanently lit picture of a car with skid lines underneath it as well. This means that DSC recalibration is required.

You do the calibration by following this procedure (as noted in the manual):

1) Turn the ignition to on (but do not start the car).
2) Moving the steering wheel full lock left then full lock right.
3) Note that the DSC OFF and car skid light have gone out
4) Turn the ignition to off
5) Turn ignition back to on, and verify that the DSC related lights in the instrument cluster go out as normal.

If the lights remain on after doing this then take it to a dealer - and drive carefully! The car will not behave on the road as you would normally expect.

Watch the following two interesting videos that show all the tehnical data about the Renesis 1.3 litre unique engine, and also help to understand how the Rx8 wankel rotary engine works. It shows the oil cooling system for both rotor chambers, the fuel circulation and the ignition system.

The second video shows how the compression test can be done on both versions, the 192 bhp manual 5 gears, and the high power version 231 bhp 6 gears. This test is very important in case you decide to buy a Second hand Rx-8 , because it’s the most accurate method to find the usage of the rotary engine.

Launched in 2003, the Mazda RX-8 hit the global market with a serious bang. The RX-8 has won more than 50 global awards since its release including 2003 RJC Car of the Year in Japan Australia’s Wheels magazine’s Car of the Year for 2003, 2003 International Engine of the Year, 2004 Singapore Car of the Year, the 2004 U.S. Best Sports Car and UK Car of the Year 2004. It was also named on Car and Driver magazine’s Ten Best list for 2004, 2005, and 2006. All together, Mazda has sold nearly 167,000 RX-8s around the world.

For 2009, Mazda will further the evolution of the four-door, four-seater sports coupe, giving RX-8 a refreshed exterior and interior design, enhanced performance and a new R3 sport package for the ultimate driving enthusiast.

Unchanged is the core of the RX-8 – a high-powered, lightweight and perfectly balanced machine powered by the world-renowned twin-rotor RENESIS rotary engine.

“Dating back to the Cosmo Sport released in 1967, every sports car ever developed by Mazda has had the same fundamental mission – to provide car lovers with a vehicle that’s fun-to-drive, exciting to look at and easy to own,” said Tetsu Nakazawa, Mazda North American Operations’ RX-8 vehicle line manager. “The 2009 RX-8 embodies that mission, conveying the unique and distinctive Mazda brand DNA to the fullest.”

To enhance the highly successful RX-8, Mazda engineers focused on innovation in three key areas for the 2009 RX-8: styling, performance and packaging. “The enhancements made to RX-8’s exterior, interior, packaging and performance takes it to a whole new level of visual and driving excitement,” remarked Nakazawa.

RX-8 was designed with an athletically sculpted exterior that provides a sense of originality that’s unrivaled in the marketplace today. For 2009, RX-8 receives design enhancements that are meant to freshen the styling and give RX-8 a new look, without impairing the basic design theme. Refinements for the 2009 model year include restyled front and rear bumpers and front fascia; sporty, high quality finish front and rear headlamps; and larger exhaust pipes (now measuring 90 mm across). The 2009 RX-8 also offers a new five-spoke wheel design featuring a symbolic and sporty design reminiscent of the rotary engine, with different arrangements for each wheel size.

Taut muscular lines give RX-8 the liberating look of an athlete in motion. The muscular styling maintains classic sports car proportions while adding a Zoom-Zoom edge that is unmistakably Mazda.

The RX-8’s unique “freestyle” four-door design is proof that a true sports car does not need to sacrifice space or convenience for performance. The advanced design of the rear-hinged rear doors, provides a large door opening, allowing adult-sized passengers to easily enter and exit the vehicle. This design is also advantageous when securing a baby or a small child in the back seat. With a spacious rear seat area providing ample passenger room for four full-size adults, and enough trunk space for a weekend’s worth of luggage, this sports car proves its versatility.

The RX-8’s exterior styling presents a genuine sports car form, while the interior boasts a comfortable and intelligently designed cabin. Minor enhancements were made for the 2009 RX-8 to provide a simple and functional interior design that supports driving pleasure. The center IP shape was redesigned to give a feeling of dynamic movement, a variable red-zone was added to the tachometer that will rise as the engine comes to operating temperature and a new steering wheel and redesigned front and rear seats are also added.

An extremely low cabin floor allows the seats to be mounted low in the chassis, which, along with a low instrument cluster and hood, allows a low center of gravity and allows outstanding occupant visibility. Mazda designers concentrated on the shape of the front seat backs and the rear seat cushions to ensure adequate rear-seat knee room. Front seat slide-rails are positioned to allow maximum leg room for rear-seat passengers.

The rotary design element is incorporated through the interior of the RX-8 in creative ways, appearing in the seats. The stylish cabin also evokes a sense of luxury and high-end quality. Mazda’s design team examined every aspect and component of the interior and has created an elegant, driver-centric atmosphere. Through the use of advanced ergonomic research, Mazda engineers determined that improper seating posture is a major cause of driver fatigue and built in optimum support in the front seats to help offset discomfort. In addition, the color of the RX-8’s instrument cluster was developed to reduce eye fatigue and strain.

The 13B REW twin-rotor engine fitted to all third generation RX-7s can trace its origins to 1974 and the RX-4.

A lot has changed through the years of development. As fitted to the 1974 RX-4, the carbureted 13B produced 127 HP @6500 rpm and 128 ft-lbs of torque @4000 rpm. Later versions employed a unique sequential twin turbo system to produce 255 HP at 6500 rpm and 217 ft-lbs of torque @ 5000 rpm. Torque output was increased throughout the rev range with as much as 181 ft-lbs available at a low 2000 rpm. Maximum RPM has been raised to 8000 and the rotor’s compression ratio of 9.0:1 necessitated premium grade unleaded petrol.

The engine inherited the basic 13B geometry and epitrochoidal dimensions of 654 cc for each of its two rotors. However in 13B REW configuration, many of the rotary’s major mechanical and electronic systems received extensive modification. Among the many internal upgrades were a thin wall cast-iron rotor with fully machined combustion recesses to ensure uniform combustion. Apex seal slots were hardened to resist wear. Modifications have also been made to the aluminum rotor housing around the “hot spot” spark plug area for more coolant flow. The engine’s induction, exhaust, cooling and lubrication were modified or redesigned when compared to the series V RX-7 engine.

Cooling and lubrication are vitally important to an engine producing the power of the 13B REW. Both rotors are kept cool by splashing them internally with oil. Internal lubrication is via an electronically controlled metering system that reduces oil consumption by 25-50 percent compared to the previous method of supply to the intake and trochoid chamber combined.

Lubrication is fed under high pressure to the eccentric shaft bearings via a high-efficiency multi port rotary pump. The heated oil is then sent through two oil-coolers (one in each of the nose vents) before being reused. A lightweight aluminum and plastic radiator, fully shrouded and slanted sharply to lower the Mazda RX-7’s nose was fitted up front. Maintaining the correct temperature is a pair of three stage thermo fans and keeping the flow is a lightweight aluminum water pump.

The fire in the new 13B REW was supplied by the world’s first volume-production sequential twin turbocharger system. It produced more power than a conventional twin turbo setup where both turbos boost at once, and suffered far less turbo lag. The advantage of the sequential system was the ability to utilize a small and large turbo at the same time. At low rpm the 51mm diameter turbo with its “impact” blade design spools very quickly, providing boost from as low as 1500 rpm. At a calculated point the second 57mm diameter turbo is switched on, giving full boost all the way to the 8000 rpm redline.

A major downfall of other sequential twin turbo systems is the transition from single to double operation. This is due to the second turbo not spinning fast enough when it’s called on, resulting in the engine “coughing” momentarily.

Mazda solved this problem by spooling the second turbo to a pre-boost speed of around 100,000 rpm with exhaust gas circulation. When the time comes for the second turbo to cut in, a bypass valve is shut to “surge” the compressor, spinning it to over 140,000 rpm. This then assures a smooth coupling with the primary turbo. Once at the required speed, it receives a full share of exhaust flow to add its boost with the primary turbo.

The twin turbo chargers are fitted to a “dynamic pressure” cast iron exhaust manifold shaped to minimize the gap between the exhaust ports and the intake of the turbos, improving boost by as much as 35 per cent. The 13B-REW was fitted standard with an air-to-air intercooler mounted above the radiator with separate ducting.

The ECU controlled fuel injection system uses air density measurement instead of the common air flow metering to allow a smoother air flow and more precise fuel management. Coupled with the precise shape of the plenum chamber and inlet tract length, the added benefit of the increased power and throttle response is the additional improvement in fuel consumption for the city/highway cycle.

Two injectors per rotor look after the varied engine loads. The primary injector takes fuel from the side of the injector body instead of the top. The resulting reduction in fuel travel through the injector body gives a quicker and more precise engine re-start after high speed/load applications. The secondary injector is mounted upstream of the inlet tract operating during mid and high rpm. Change over to twin injector operation takes place around 2,750 rpm or lower if the load commands it.

Ignition timing of the four platinum tipped spark plugs (two per rotor) is controlled via the ECU which will automatically retard timing if detonation is detected. Lightweight high energy coils are utilized to take advantage of the rotary’s unique combustion characteristics.

Ensuring the emissions are lower than required, the use of a double-skin exhaust manifold and high performance three way catalysts gives the lowest flow resistance available and lower emissions. A single muffler is used for noise reduction and the whole system weighs considerably less compared to the 3rd generation RX-7.

RENESIS - The Future of the Rotary Engine.
For rotary engine enthusiasts, the next exciting phase in the great engine’s history has already begun. At the Tokyo Motor Show in October 1999, Mazda unveiled the RX-Evolv, a concept vehicle which later evolved into the MAZDA RX-8 four-door, four-seat sports car unveiled in January 2001 at the North American International Auto Show (NAIAS) in Detroit. The Evolv and the MAZDA RX-8 shared many advances in common, not the least of which was the latest version of the rotary engine called “RENESIS.”

The MAZDA RX-8 with its RENESIS rotary engine will make its debut in 2003.

When developing the RENESIS, Mazda’s engineers aimed to retain power output on a par with the turbocharged 13B-REW, the rotary engine that powers the Mazda RX-7, while offering improved fuel economy and reduced emissions.

Side Intake and Exhaust Ports
Unlike previous mass-production rotary engines, which employed side intake ports and peripheral exhaust ports, the naturally aspirated RENESIS has intake and exhaust ports in the side housings. This configuration eliminates overlap between the opening of the intake and exhaust ports, enhancing combustion efficiency. The intake ports are 30% larger and their timing has been changed to make them open sooner than in previous designs. Moreover, the exhaust ports open later, resulting in a longer power (expansion) stroke and providing radically improved heat efficiency.

At the same time, the RENESIS uses a six-port induction (6PI) design, in which each rotor employs three intake ports, and a variable intake timing mechanism. Under this system, dedicated high-speed intake ports begin to operate when the engine operates at high-rev levels. This makes it possible to use the intake’s dynamic effect at high and low speeds to maximize compression efficiency.

Unlike the single peripheral port per rotor of previous designs, the RENESIS uses two exhaust ports per rotor. This produces a combined exhaust port opening area nearly twice as large and results in a substantial reduction in exhaust resistance.

The rotors have also been made lighter for better performance at high-rev levels. The rotors used in the RENESIS weigh approximately 14% less than those used in the engine that powers the Mazda RX-7, which is sold in Japan.

These enhancements provide high output rivaling the power of turbocharged rotary engines with linear power characteristics from the low- to the high-rev range.

*These results should
be used for comparison only, and verification should not
be attempted on public roads. Actual performance results may vary depending
on specification of vehicle, road and environmental conditions,
testing procedures and driving style.